Corrosion of metallic surfaces used in equipment and workpieces in water systems in the food and beverage industry is a significant problem. Industrial processes exist for the purpose of sterilizing equipment and reducing corrosion in a hydrostatic environment. For purposes of this disclosure, a hydrostatic environment is a state at which there is equilibrium between a liquid and the pressure of steam exerted by liquid at rest. In a hydrostatic cooker, this principle is applied in the context of a system where pressure of saturated steam in a sterilizing zone balances the weight of water in adjacent zones. The temperature in the sterilization zone is directly related to the pressure of the saturated steam. The sterilization process can be controlled by varying the pressure, and thus the temperature, in the sterilization zone.
A typical hydrostatic system is used for treating workpieces such as sterilizing cans for foods or beverages. These systems usually consist of four or more stages, each with a height of several meters. The first stage serves as a preheating section, in which a hydrostatic column serves as an inlet section. The second stage includes a sterilization zone, where a workpiece is heated to the requisite sterilization temperature using steam. The third stage includes a hydrostatic cooling section that the product ascends through after leaving the sterilizing zone. At this stage, with the pressure gradually decreasing and the product gradually cooling, spray water jets may be used to provide additional cooling. At that the final stage, the cooling cycle is completed in the fourth tower at atmospheric pressure.
Examples of hydrostatic systems include retort systems such as illustrated in FIG. 1. Retort systems sterilize canned products after they are sealed. Hydrostatic sterilizers use a column of water to maintain the pressure in the sterilizing zone. The conveyor line of containers enters the sterilizer at the top of the water column and travel downward to the bottom of the steam dome, which is the heart of the sterilizer, as seen in FIG. 1. Once in the steam dome, the containers make several passes through the steam zone before being conveyed again to the bottom of the steam dome, into the exit water column and out of the sterilizer. The water column keeps steam inside the steam dome and provides the necessary pressurization. The height of the water column, approximately 35 feet, usually mandates that the equipment is located outdoors.
The containers are carried through the cooker on mild steel bars which are mounted at each end on a continuous chain. The chain loops up and down through a pre-heat leg where the cans are contacted with hot water at about 170 to 180° F. through a steam chamber and finally through a cooling leg before being spray cooled with cooler water at about 65-85° F. The total process time varies between 10 and 60 minutes, and the temperature in the cooker ranges from 90 to 212° F.
Hydrostatic systems in the food and beverage industry employ processes that generally include filling a container with food or beverage product, sealing it and then heating it to destroy any residual microorganisms. After the container is heated for the required period of time, it is cooled to ambient temperature either for storage or shipment. During this process, the corrosion of containers and the cooker must be carefully controlled through a properly designed chemical treatment program. Currently used chemical treatment methods for controlling the vapor phase and liquid phase corrosion or staining on containers in hydrostatic systems are not very effective.
Moreover, containers used in hydrostatic systems are typically made of aluminum or some alloy thereof. Conventionally, in order to resist corrosion of the container, the container is coated with a protective substance, such as tin, through an electrolytic process during manufacture of the container. Likewise, the container may be coated with a polymer coating such as an epoxy coating.
Additionally, there are various issues associated with the operation and maintenance of equipment used in a hydrostatic environment including scale and corrosion of in-feed and exit legs, can abrasion, corrosion, staining, vapor phase corrosion, microbiologically induced corrosion, and the like. Poor corrosion control in a hydrostatic cooker leads to excess corrosion rates of about 1.3 mm per year (mpy) and can be as high as 50 mpy. An aggressive processing environment and poor quality makeup water can exacerbate the issues.